Image compression is one of the key compression technologies in today's network centric world. There are various image compression techniques available. Among these techniques, JPEG is the standard technique for compression of still images. However this standard has many limitations. JPEG2000 (ISO/IEC 15444) is the new standard for image compression catering to different needs of various applications. This standard is much more advanced than JPEG and is rich in features.
Brief of JPEG 2000 Coding
FIG. 1 of the accompanying drawings shows the basic components involved in the JPEG 2000 coding of images.                1. FIG. 1a of the accompanying drawings, shows an image comprising color components. (For example R, G, B or Y, U, V etc).        2. The Discrete Wavelet Transform (DWT) is applied on each of the color components to generate frequency bands and resolution as shown in FIG. 1b. When the DWT is applied only once, LL, HL, LH and HH bands are generated. The DWT can be applied again and again on LL bands. Shown in FIG. 1b of the accompanying drawings is a two level application of DWT. Each of LL, HL, LH and HH are called as sub-bands. These sub-bands together form a resolution.        3. Thus LL2 is resolution level 2, HL2+LH2+HH2 is Resolution level 1 and HL1+LH1+HH1 is resolution level 0 (i.e. original image).        4. The sub-bands are further subdivided in code-blocks (as seen in FIG. 1c). These are the basic building blocks of the JPEG2000 compression scheme. Each sub band is partitioned in the code blocks.        5. The code-block size is flexible.        6. JPEG 2000 stream is composed of coefficients of the code-block coded in efficient manner to give desired bit-rate and compressed visual quality.        7. The code blocks are coded in bit plane fashion starting from most significant bit plane and ending with the least significant.        
FIG. 2 of the accompanying drawings shows the hierarchy of the JPEG 2000 bit stream from the basic blocks point of view.
The decoding of the JPEG 2000 happens in the sequential order as outlined below in brief.                1. The bit planes of the code block are first decoded.        2. The code-blocks from bands are collected.        3. Once all the code-blocks from the bands of a particular resolution of a particular color component are formed, Inverse Discrete Wavelet Transform (IDWT) can be applied to get resolution of that color component.        4. This is repeated for other sub-bands of other resolutions and color components.        5. Finally the color components are combined together to give the decoded image.Corruption of the JPEG 2000 Compressed Bit-stream        
FIG. 3 of the accompanying drawings shows the block schematics of the image transmission and reception. As it is well known that the wireless channel (or any channel for that matter) is error prone. The additive and multiplicative noises of channel will result in corruption of bits. When such a stream is decoded there could be a loss of picture information.
Effect of Corruption of JPEG 2000 Bit-stream
Channel errors lead to the wrongly decoding of code-blocks. Such code blocks are detected and replaced by zeros to form lost code-blocks.
As shown in FIG. 4a of the accompanying drawings, the code blocks from each of the color component in LL3 band is lost. For the sake of illustration the code-block at different locations is considered to be lost. When the IDWT is taken on this resolution the size of the lost code-block grows, this is due to the nature of Inverse Discrete Wavelet Transformation (IDWT). This is shown in FIG. 4b of the accompanying drawings. Another level of IDWT gives back the original image components as shown in FIG. 4c of the accompanying drawings. So it can be seen that a loss of small size code-block at LL3 band leads to the loss of a major area in the reconstructed (decoded) image.
To remove these errors there is a demand for concealment of errors. These errors are detected by known methods of detection. Detection of errors is done using the error resilient features of the JPEG 2000 Bit-stream. This involves resetting of the arithmetic decoder, use of packet header and so on. By means of these techniques the errors are localized to code-blocks or adjacent code-blocks and the location of these lost code-blocks and their sizes of the lost code-blocks are known by these detection methods.
Once the errors are detected various methods have been applied to conceal these errors. Once such method is the Bi-directional Interpolation Algorithm.
This is the simplest technique and is based on interpolation of boundary coefficients in the layer surrounding the lost block. The top-left lost block coefficient is x0,0 and the bottom-right lost block coefficient is xM−1,N−1. λ=0.5 is used.
FIG. 7 of the accompanying drawings shows how the algorithm works. Only the light gray coefficients surrounding the lost block (in black) are used for concealment of all coefficients within the block. Specifically the coefficient shown in light gray at location xi,j is concealed from the four light gray coefficients with coordinates xi,−1,xi,N,x−1,j and xM,j as described by the equation below.
The equation that describes bi-directional interpolation isxi,j=λ{αjxi,−1+(1−αj)xi,N}+(1−λ){βix−1,j+(1−βi)xM,j} for 0≦i≦M−1, 0≦j≦N−1where                1. M is the height of lost block of coefficients        2. N is the width of lost block of coefficients        3. αj=1−(½N+j/N) and        4. βi=1−(½M+i/M) are the distance operators.        
In the prior art various methods have been suggested for concealing of these errors. For instance:
U.S. Pat. No. 6,137,915, discloses an Apparatus and method for error concealment for hierarchical sub-band coding and decoding which conceals error in bands of wavelet using other band coefficient at the same location. This operates on band level error concealment and uses the coefficients in the other sub-bands at the same resolution to conceal the corrupted coefficients. However, this method does not use the bit streams properties of JPEG 2000 and therefore does not use the inter-color-component correlation. The scheme in this patent employs a method to conceal error in the lost sub-band of DWT transformed image using other sub-band data. This is therefore a generic scheme and does not work on JPEG 2000 as such. Also this scheme considers the error concealment in the higher sub-bands only.
Again, U.S. Pat. No. 6,526,532, discloses a Channel Error correction apparatus and method” deals with the errors in the sub-bands of DWT. This technique uses cross-band redundancy to conceal the error. First the error is estimated for location and amplitude and then concealed. There are other error concealment techniques, which are however not applied on DWT image but are depended on the information present in the image itself The method of this patent again does not use the inter color component correlation.
Zeng, W., Liu, B., Geometric-Structure-Based Error Concealment with Novel Applications in Block-Based Low-Bit-Rate Coding, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 9, No. 4, June 1999 identifies the geometrical structures in the lost region. This method implements directional interpolation. The edges going in the lost region are identified and interpolated along with the direction of the edges.
Sofia Tsekeridou, Ioannis Pitas, “MPEG 2 Error Concealment Based on Block Matching Principles”, IEEE Transactions on Circuits and Systems for Video Technology, Vol. 10, No 4, JUNE 2000. discloses a method which is suitable for MPEG-2 video but some of the specified techniques could be implemented on images as well. This reference talks about Spatial Anisotropic Diffusion (SAD) Error Concealment method. SAD depends on the diffusion of the surrounding image contents in the lost area.
Finally, Sun, H., Kwok, W., “Concealment of Damaged Block Transform Coded Images Using Projections onto Convex Sets”, IEEE Transactions of Image Processing, Vol. 4, No. 4, April 1995. uses spatial interpolation algorithm. The neighboring blocks are used to classify the lost block as monotone area or edge area and incase of edge area an angle for the edge is also computed. Based on this the lost block is connected for smoothness, edge etc.
None of these cited references use the inter color component correlation of JPEG 2000. The advantages of using this correlation is as follows:                1. Information present in the sub-band of the other color component can be used to conceal the error. In case of RGB image there is strong correlation in the color components.        2. As wavelet transformed image splits the details present in the image in various resolution and subbands, the error concealment from the same color component but different resolutions and subbands may not give effective concealment.        3. In JPEG 2000, each of the color component is coded independently so chances of corruption of the code-block at the same location in different color component is somewhat lowered.        
An object of this invention is to provide Error concealment in JPEG 2000 which mainly deals with estimating the lost regions in a Wavelet Transformed image and works in Wavelet Transform Domain. This will results in the optimum error concealment and therefore, the method is applied before taking Inverse DWT.
One object of the method of this invention is that it operates on corresponding to the lost block region at the lowest resolution level in wavelet transform domain.
Another object of this invention is that it makes use of coefficients from other color components.
Yet another object of this invention is that it preserves all information from the JPEG 2000 compressed stream.
Still another object of the invention is that, the method is applied only over the lost coefficients in LL subband representing the lowest resolution level. So the information available in all other subbands is preserved. Error Concealment after Inverse DWT would have resulted in loss of this information at regions in reconstructed image.
Another object of this invention is to apply the method of the invention only over the lost coefficients in LL sub-band representing the lowest resolution level. The dimensions of LL sub-band representing the lowest resolution level are smaller than the dimensions of reconstructed image, and therefore another object achieved in accordance with this invention is that computations to be performed are reduced drastically.
Still another object of this invention is to perform concealment only for the blocks lost in LL sub-band representing the lowest resolution level and ignoring other lost blocks taking into account the fact that this sub-band is much more important as compared to any other sub-band.
Another object of this invention is to perform error concealment in one sub band in manner such that information available in all other sub-bands is preserved.
A final object of this invention is to provide a complete error concealment scheme, which selects an appropriate approach for concealing each lost code block, based on the values of coefficients in the neighborhood of the code block under consideration, size of the block (number of coefficients) lost and/or coefficient values in the corresponding region in other sub-bands.
To meet with the aforesaid object there is provided in accordance with this invention, a method and apparatus which takes care of the principle that image compression leads to removal of the redundancies in the image in order to represent the image more compactly resulting in lesser memory disk space and at the same time error concealment needs some redundancy in order to conceal the errors.
This invention also provides a method and apparatus which uses the inter color component redundancy in JPEG 2000.